1. Field of the Invention
The invention relates to a switching device comprising at least one feeding circuit, which feeding circuit is provided for DC power supply to at least one subscriber's terminal unit.
2. Discussion of the Related Art
If, in a fault condition of a DC power supply to a subscriber's terminal unit, excessive currents occur i.e. inadmissibly high supply currents which are caused, for example, by an overvoltage resulting from static charging or a short-circuit on the subscriber lines of the subscriber's terminal unit, there is a danger of a subscriber line being damaged (specially a fire on a subscriber line is to be avoided), or components of the switching device being damaged. For this reason, the feeding circuit is to include a protection against excessive currents.
British Patent Specification GB 2 186 763 A has stated for this purpose a protective circuit which includes a shunt as a protection against overvoltages on its output, which output is coupled to a subscriber's terminal unit. Overcurrents are sensed by two transistors which trigger a monostable in the case of overcurrent, so that this monostable turns off via a further monostable two field-effect transistors working as supply transistors. The field-effect transistors are thus used as controllable resistors and as switches via which a supply current for the subscriber's terminal unit flows.